A typical electrostatographic printing process includes a development step whereby developing material including toner or marking particles is physically transported into the vicinity of a latent image bearing imaging member, with the toner or marking particles being caused to migrate via electrical attraction to the image areas of the latent image so as to selectively adhere to the imaging member in an image-wise configuration. Various methods of developing a latent image have been described in the art of electrophotographic printing and copying systems. Of particular interest with respect to the present invention is the concept of forming a thin layer of liquid developing material on a first surface of a first member, wherein the layer has a high concentration of charged marking particles. The layer on the first member is brought into contact with an electrostatic latent image on a second surface of a second member, wherein development of the latent image occurs upon separation of the first and second surfaces, as a function of the electric field strength generated by the latent image. In this process, toner particle migration or electrophoresis is replaced by direct surface-to-surface transfer of a toner layer induced by image-wise fields.
For the purposes of the present description, the concept of latent image development via direct surface-to-surface transfer of a toner layer via image-wise fields will be identified generally as contact electrostatic printing (CEP). Exemplary patents which may describe certain general aspects of contact electrostatic printing, as well as specific apparatus therefor, may be found in U.S. Patent Nos. 4,504,138; 5,436,706; 5,596,396; 5,610,694; and 5,619,313.
It is desirable that the aforementioned layer of liquid developing material be provided in a very thin and very uniform layer that exhibits a high proportion of solids, that is, having a high solids content. Even more desirable is such a layer exhibiting the following advantageous characteristics: a selectable, uniform thickness, preferably in the range of 3-10 microns; a high solids content, preferably in the range of 15 to 35 percent solids; and an accurately metered mass per unit area on the order of 0.1 mg per cm.sup.2.
The intuitive and conventional approach is to attempt the formation of such a layer by direct application of liquid developing material having a high solids content. However, due to the very complicated rheological behavior of a liquid developing material having the requisite high solids content, such direct application of a supply of such liquid developing material to a receiving member typically does not achieve a layer having the aforementioned desirable characteristics. For example, the resulting layer has been found to exhibit a variable thickness and a non-uniform mass per unit area, which renders the layer generally unsuitable for most contact electrostatic printing applications.